I would like to design and build a tunable lowpass filter. I would like to adjust the cutoff frequency from 1 MHz to 500 MHz. Requirement on the resolution would be something like 10-50 MHz, obviously, it's not that critical.
I am going to send a programmable clock signal through this filter and want to reduce the high frequency harmonics. I am only using the clock signal as a simple way to get a programmable sinusoidal signal, hence the need for the filter. I was considering using the ADF4110, ADF4350, or one of the Silicon Labs Si570/571 as the programmable clock. I was told that a tunable lowpass filter over this range may be tricky, so I thought it best to consult the StackExchange. I am open to any suggestions.
Ultimately, what I'm trying to accomplish is get a sinusoidal signal that will sweep from 1-500 MHz.
Best Answer
Tunable filters over this sort of range are really, really difficult to do. For reals.
In high end network analysers you will find a special type of filter called a YIG filter (low end ones often don't use them because of their price) or a sweep oscillator based on a YIG VCO. Your best bet would be to just make 10 lowpass filters and switch them in with relays (or use relays to change the capacitance). You'll note that older signal generators do a lot of clicking when you crank the frequency up. If all of the HP RF boffins couldn't work out a good way of making tunable filters then I think you may struggle.
But to be honest, I am not sure why you are doing this to generate a sine wave. Just go DDS! Any modern signal and/or function gen will use DDS for this frequency range. You can buy off the shelf chips that go way above what you need; the AD9914 does <0Hz to 1.4GHz sine waves.